This invention relates to the measurement of the mass of particulate or other forms of matter contained within a medium such as air or other fluids. It concerns itself both with a method and apparatus for making such measurements.
This invention represents an improvement on the method and apparatus described in U.S. Pat. No. 3,926,271, the contents of which are hereby incorporated by reference. Hereinafter, said patent shall be referred to as the "microbalance patent."
The apparatus described in the microbalance patent can be used for the measurement of the mass of particles and other matter contained within a gaseous or liquid medium through the use of impaction or other techniques wherein the matter is deposited on substrate 30, shown in FIG. 1 thereof. The impaction technique involves directing a flow of the medium on the substrate so that matter contained in said medium will be deposited on the substrate. This process, while perfectly adequate for many applications, is nevertheless subject to certain shortcomings. Obviously, only a certain percentage of the matter contained within a medium will be deposited, the remaining matter either bouncing off the substrate or being carried away from it or around it in the local turbulence created at the surface of the substrate. It would, of course, be desirable if a higher percentage of the matter whose mass is to be determined could be deposited on the oscillating end of the elastic element 4 described in the microbalance patent.
In the present invention, this objective can achieved through the attachment of a filter to the oscillating end of the elastic element and passing the medium containing the matter through the filter. In the preferred embodiment, the oscillating element is provided with a channel running from its oscillating end through its base and the filter is attached so as to cover the opening of the channel at the oscillating end. The medium is preferably drawn through the element so that it will pass first through the filter and then through the element. The associated structures and apparatus described in the microbalance patent and its method of operation would remain the same. Of course, equivalent structures and apparatus could be substituted.
Basically then, this invention concerns itself primarily with a modification of the invention disclosed in the microbalance patent wherein the modification consists in substituting a filter for the substrate 30 and providing means for circulating the medium containing the matter whose mass is to be determined through the filter. As that is done, the mass can be determined, as disclosed in the microbalance patent, by monitoring the changes in resonant frequency as matter becomes entrapped in the filter. For liquids, real time measurement will generally not be feasible because the passage of liquid through the oscillating element has an obvious damping effect and because it is generally necessary to remove the liquid from the filter after the deposition of matter therein by evaporation of the liquid or some other process.